The moons of the Solar System

The recent discovery of another moon orbiting Pluto -- taking
the number of moons under the influence of the dwarf planet to five
-- highlights that beyond the eight planets there are many varied
and interesting celestial bodies and discoveries are still being
made in our Solar System.

Not counting the five moons around Pluto, now classified as a
dwarf planet, 172 moons have been found orbiting the eight planets.
There may well be more to discover -- two more small moons (2km --
3km diameter) were discovered orbiting Jupiter in 2010 and
confirmed in June 2011 by the International Astronomical
Union, as reported
by Sen.

We have discovered that not all moons are dead cratered worlds.
Our exploration of the Solar System has instead found a diverse
range of worlds, some with subsurface oceans, others geologically
active with giant geysers and volcanoes, and Titan, with a thick
atmosphere.

Our own Moon is still the subject of exploration,
including NASA's twin GRAIL
probes that are currently in orbit. Across the planetcompanies
are competing to send a robot to the Moon to win the
Google Lunar X PRIZE. Others are offering manned missions - Space
Adventures and Excalibur Almaz will fly you to the Moon for US$150m
per astronaut.

Further out at Mars are two small moons, Phobos and Deimos.
Sadly in 2011 the Russian Phobos-Grunt probe failed in the early
stages of its mission to the larger of the red planet's two moons.
Phobos orbits closer to its parent planet than any other moon -- at
just 6,000km above the Martian surface. The rocky moon, which is
the larger of the two, is still small measuring
approximately 27 by 22 by 18 km in diameter.

Around the Solar System's largest planet, Jupiter, are the most
moons. Jupiter's four largest moons -- Io, Callisto, Europa and
Ganymede -- are known as the Galilean moons after Galileo Galilei
who discovered them in 1610.

Io is the most volcanic place so far discovered in the Solar
System. As a small frozen moon in the outer Solar System its
geological activity came as a surprise to scientists, who reason
that Io's internal heat is generated from a complex gravitational
tug of war, the small moon being subjected to the gravitational
forces of both Jupiter, Europa and Ganymede. The effect of this
gravitational interplay is that Io is squashed and stretched
creating an internal heat source. Io has huge volcanic
eruptions that cover the surface in sulphur giving the volcanic
moon it its yellow colour.

Callisto, Europa and Ganymede all have internal oceans where
microbial life may exist. These moons will be explored by theEuropean Space
Agency's Jupiter Icy Moons Explorer which will arrive at
Jupiter in 2030. We already have some knowledge of these moons
gathered by NASA's Galileo probe between 1995 and 2003.

Jupiter's moons range in size from just 2km diameter to
Ganymede, the largest moon in our Solar System with a diameter
of 5,260 kilometres -- larger than the planet
Mercury. Ganymede has its own magnetic field which could be
explained either by a liquid iron outer core around its solid heart
or an ocean beneath the surface.

Callisto is the outermost of the four Galilean moons,
orbiting the gas giant at 1.88 million km. It is Jupiter's second
biggest satellite and the third biggest in the Solar System with a
diameter of more than 4,800km.

Europa has fascinated scientists because its surface does not
appear cratered like other moons, but instead has an icy white
surface marked with cracks that resemble the ridges of ice found in
the Arctic. Measurements of a magnetic field add to evidence
that a salty subterranean sea is wrapped around the moon which
could be more than 100km deep and a prime place to search for alien
life. Europa is 3,140 km in diameter and lies 671,000 km from
Jupiter.

Further from the Sun we come to Saturn where 62 moons have so
far been found. Around Saturn, worlds that have created
particular interest are Enceladus and Titan.

Carolyn Porco told Sen "No other planetary system in our solar
system is more phenomenologically rich than Saturn's.
The planet's moons alone -- from the tiny ring moons whose
origins and development are intimately tied up with the rings,
to bright icy Enceladus harboring the solar system's most
accessible habitable zone, to planet-sized, haze-enshrouded
Titan dotted with seas of liquid organics -- offer the most
diverse range of planetary environments in orbit around any
one planet. In having this richness before us, our mission on
Cassini has been unique."

Titan is the second largest moon in the solar system and
Saturn's largest moon. Titan has a dense atmosphere made primarily
of nitrogen and methane. The images of Titan sent back by
Cassini show this atmosphere as a thick blue line surrounding the
moon. Under the atmosphere we have explored the surface of Titan
with the Huygens probe which landed on the surface in 2005,
taking pictures as it descended through the atmosphere. The
pictures taken by Huygens and Cassini have revealed a landscape
similar to one that you can find on Earth, with mountains, lakes
and river beds covered in rounded stones and pebbles that had
clearly had their surfaces smoothed by flowing liquid.

Enceladus is a very small icy moon, only about 500 kilometres
across. Its icy surface is highly reflective of the sunlight that
reaches it making it extremely bright. Enceladus is geologically
active and in the south pole region are over 90 jets that
spray water vapor, icy particles, and organic compounds into
space. Cassini has flown several times now through this spray and
found that there is salt in the icy particles. Carolyn Porco
believes Enceladus is the most promising place in the solar system
for an astrobiology mission to search for microbial life.

Beyond Saturn, Uranus has 27 moons, named after characters from
Shakespeare's plays. Oberon and Titania are the largest and were
discovered by the astronomer who discovered Uranus itself --
William Herschel. Three more moons were discovered before the
Voyager 2 spacecraft identified ten further moons. The inner moons
appear to be half water ice and half rock. Since Voyager 2 some
very small moons -- 12 to 16 kilometres across -- have been
discovered by Hubble.

Finally, the ice giant Neptune has 13 satellites, the largest
being Triton, discovered in 1846 by brewer and astronomer William
Lassall. The third largest moon, Nereid, was discovered in 1949 by
Dutch astronomer Gerard Kuiper. The second largest moon, Proteus,
is dark and close to Neptune and was discovered along with five
other satellites by Voyager 2. Further moons were observed from
Earth in 2002 and 2003 taking the known total to 13.

Beyond Neptune lies Pluto, with its five moons, and then the
Kuiper Belt, ensuring there are a vast number of celestial bodies
orbiting our star.

Original story (c) Sen. More space content from sen.com.
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